OBPI or Erb’s palsy is a weakening or paralysis of the upper arm caused by brachial plexus injury (stretch, rupture, or avulsion), followed by a muscle paralysis (contracture in adduction), with severe repercussions on the movement of the shoulder joint (internal rotation) and progressive glenohumeral joint deformity (retroverted and dysplastic glenoid, humeral head epiphysiolisys, down-sloping acromion, elongated and vertical coracoid).

The damage entity is related to the extension (C5-C6 are generally involved, sometimes in combination with C7 [1]) and the severity of the nerve damage.

The radicular injury, more often C5-C7 usually post-ganglionic, leads to muscle weakness in abduction, involving deltoid and supraspinatus, and external rotation, due to infraspinatus deficiency; therefore the shoulder appears adducted and internally rotated, with a consequent dysplasia and dislocation of the humeral head over time [2].

The primary cause of nerve injury lies in the abnormal mobilization of the baby’s head, during childbirth not properly performed; however, also with a cesarean section brachial plexus injury may occur [3].

The right upper limb is the most often involved since the left occipital anterior vertex presentation is the more frequent [1].

Patients with OBPI may compensate for limited glenohumeral motion through their scapulothoracic articulation [4].

The literature reports various data regarding the incidence of OBPI all over the world. It varies from 0.3 to 4 per 1000 live births [4]; between 1988 and 1997, at the Academic Medical Center in Amsterdam, the incidence of OBPI was 4.6 per 1000 [5].

Muscle strength recovery seems to occur from 76.2 to 90% of patients, although a residual contracture of the shoulder is frequently observed [5].

These patients generally envelop a degenerative disease of the shoulder at a younger age than in the general population [6]; actually, no data are known about the evolution of cases towards symptomatic osteoarthritis, probably due to their low number or to the difficulty of shoulder arthroplasty surgery.

The therapy mainly consists of conservative treatment and, when chronic and uncontrolled painful end-stage osteoarthritis occurs, shoulder arthroplasty is strongly suggested to restore a center of rotation for the glenohumeral joint, improving pain and returning a better quality of life [7]. When a glenoid erosion is present, the gold standard surgical treatment is represented by an RSA and custom-made glenoid implant, followed by a careful postoperative rehabilitation [8], focused on strengthening the anterior part of the deltoid and pectoralis major to increase arm flexion and the posterior part of the deltoid in abduction to increase external rotation [9, 10].

RSA consists of three main components: the baseplate, the glenosphere, and the humeral socket. It provides joint stabilization and pain resolution; furthermore, the lateralization of the center of rotation allows the deltoid to increase its strength during elevation and abduction, avoiding cuff deficiency-related limitations.

Because of the abnormal anatomic condition characterized by internal rotation contractures and subscapularis shortening, in OBPI patients some authors [4] prefer to leave the subscapularis unrepaired to avoid stiffness and limitation in the external rotation.

To focus attention on the needs and expectations of the patient is mandatory to outline the most suitable treatment case by case; moreover, any previous therapeutic or surgical treatment must be evaluated, to obtain the best outcome and minimum complications.

First of all, the surgical approach should require a physical examination of the neurovascular district and cervical spine; the shoulder range of motion, strength, and muscle function of the deltoid must be tested [11]. A careful radiologic evaluation (Fig. 1) of the preoperative state completes patient assessment by performing a 3D-CT (3D—Computer tomography) (Fig. 2)for the study of bone structures and an MRI (Magnetic Resonance Imaging) to evaluate rotator cuff integrity (Fig. 3).

Antero-posterior and “Scapular-Y” Xrays views of OBPI shoulder in this study

Axial and Sagittal CT scan views showing retroversion of glenoid

T2W sagittal plane MRI showing massive rotator cuff tear with abnormal arthritic erosion of humeral head

The deltoid muscle is the key to RSA functioning but many authors [12, 13] demonstrated that the contribution of scapular rhythm to shoulder elevation increases significantly in the RSA shoulders.

Shoulder motion analysis is a new technical tool to accurately analyze the scapulothoracic joint during the movement of the upper arm. Its usefulness in the field of shoulder prostheses lies in understanding the movements in the three planes of the scapula, thus they can be divided into upper and lower tilting, ante- retro-positioning, and intra- extra- rotation. In literature [14] it is explained that the RSA motion pattern in normal subjects shows a more extensive range of motion in two different planes (scapular tilt and upward rotation), with an evident trend in the range of 30–90 degrees. This means that Patients with a shoulder prosthesis have a greater scapular motion compensation compared to the healthy side. As far as we know, no previous work on RSA in OBPI patients’ scapular kinematics has been performed.

Shoulder arthroplasty in OBPI is technically demanding because of the severe deformity, which is often characterized by glenoid medialization and retroversion and consequently the high risk of prosthesis dislocation because of the imbalance of the soft tissues [15, 16]. The aim of this study is to investigate the clinical outcomes of RSA in OBPI patients and to relate a specific motion pattern with shoulder motion analysis to understand better the scapulo-toracic compensation of these patients.